1. Field of the Invention
This invention relates to a cyclic process for the removal of sulfur dioxide from industrial gases and other waste gases, such as flue gases from the burning of fossil fuels, smelter gases, spent gas from sulfuric acid manufacture, etc. . . . The invention further relates to a process of the above-described type wherein the sulfur dioxide is converted to form elemental sulfur in the processing cycle.
2. Description of Related Art
The generation of industrial gases and other waste gases is a long-standing environmental problem. Such gases, particularly flue gases, often contain sulfur dioxide. Sulfur dioxide is also liberated into the atmosphere by the burning of fossil fuels and from the smelting of sulfide ores. Other major sources of sulfur dioxide pollution include: combustion of sulfur-burning fuel, sulfuric acid manufacture, sludge acid purification, sulfur recovery from plants and pulp and paper manufacture. It has long been known that sulfur dioxide in the atmosphere is responsible for acid rain which is harmful to aquatic life, crops, forest land and other vegetation. Other deleterious effects of acid rain include discoloration and deterioration of limestone, marble and roofing slate, and respiratory infections and heart disease.
Accordingly, attempts have been made to reduce the content of sulfur dioxide in industrial gases. For example, the sulfur dioxide content of combustion emissions has been controlled to some extent through the use of low-sulfur fuels. However, such attempts have not been successful in eliminating the sulfur dioxide. Moreover, the use of sulfur fuels is often preferred or unavoidable and the use of low-sulfur fuels only affects a single source of sulfur dioxide generation (i.e., fuel combustion). The sulfur dioxide liberated from other industrial sources, described above, cannot be controlled in this manner. Thus, other methods have been developed for the control of gaseous emissions. Such methods include absorption, adsorption, condensation and chemical reaction.
In the absorption area packed columns or towers are generally used. Acidic gases such as SO.sub.2 can be absorbed using a scrub liquor such as an alkaline ammonium salt solution or an organic liquid such as dialkylaniline or diethanlolamine. One major disadvantage of such methods is that the recovered gaseous pollutants may present a disposal problem. For example, a pollutant may be precipitated in an insoluble sludge which is difficult to handle. Other drawbacks to such methods include organic compound oxidation, fire and the necessity to control scrub liquor volatility to prevent vapor loss to the atmosphere.
In the chemical reaction area, U.S. Pat. No. 3,632,306 discloses a process for the removal of sulfur dioxide from a flue gas. The flue gas is scrubbed with an aqueous slurry containing one of calcium oxide, calcium carbonate, magnesium oxide, magnesium carbonate, barium oxide or barium carbonate. A solid sulfite is precipitated and must be disposed of. No elemental sulfur is generated by the process. Other processes have been developed which result in the oxidation of SO.sub.2 to sulfuric acid. In U.S. Pat. No. 2,021,936 a flue gas is washed with a solution containing ferric ions and manganese ions. In U.S. Pat. No. 3,848,058 waste gas is contacted with an aqueous solution of ferric sulfate to form sulfuric acid.
It is an object of the present invention to provide a cyclic process for removal of practically all of the sulfur dioxide from a waste gas using a solution which is regenerated in the process.
It is a further object of the invention to provide a process of the previously mentioned type wherein elemental sulfur is formed and recovered as a product.
It is a further object of the invention to provide a process of the previously mentioned type which is economical and wherein there is no formation of waste products which present a waste disposal problem.
It is yet a further object of the invention to provide a process of the previously mentioned type wherein most of the process steps can take place at low temperatures.